Method of producing a dispersion-strengthened aluminum alloy article

ABSTRACT

Aluminium alloy articles of high strength and high resistance to temperature softening are produced by spraying droplets of selected aluminium alloy entrained in a stream of gas onto a substrate under such conditions that the metal droplets strike the substrate in a highly undercooled (supercooled) condition. On striking the substrate the undercooled liquid droplets flatten and are very rapidly chilled so that the alloying constituent is either maintained in supersaturated solid solution or is precipitated as a very fine precipitate. The deposit is consolidated by warm working. The selected aluminium alloy contains up to 25 percent of alloying constituent, which is in excess of the equilibrium solid solubility and has a low diffusion rate in aluminium. The preferred alloying constituents are one or more of Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb and Mo. Si may be added but is unsuitable by itself. Particularly satisfactory results are obtained with eutectic ternary alloys containing Ni or Si.

United States Patent Read et al.

[ 1 Aug. 19, 1975 ALLOY ARTICLE [75] Inventor? Peter John Read, South Newington.

near Banhury; Keith Graham Latimer, Greatworth; Terence David Warren Reynolds, Daventry, all of England; David Munson, deceased. late of Wruxton, England. by George Munson. administrator. Rochester, England {73] Assignee: Alcan Research and Development Limited, Montreal, Canada [22] Filed: June 2i, I973 [2]] Appl. No.: 372,204

[30] Foreign Application Priority Data June 30, I972 United Kingdom 30876/72 [52) US. Cl. 29/4205; 29/5275: 29/DIG. 39; [64/46 [5 1] Int. Cl t, B22! 3/24 [58] Field oi Search 29/5275, 527.7, DIG. 39. 29/4205; l64/46 [56] References Cited UNITED STATES PATENTS l l 28,058 2/l9l5 Schoop 164/46 X 2 l7l,599 9/l939 Reid 164/46 2,967,35l l/l9ol Roberts et al. 29/4205 3,670,4Utl (1/1972 Singer 29/5277 X 3,689,987 9/l972 Teague l64/46 X 3,696,502 lll/l972 Darling 29/5277 3 7ll,3l(l [/1973 Leeper ll7/IU5 X 3,742,585 7/l973 Wentiell l64/46 X FOREIGN PATENTS OR APPLICATIONS [349,452 4/[974 United Kingdom 29/4205 OTHER PUBLICATIONS G. Close, Metal/ ing wit/t A/untt'ntmt, Light Metal Age, March I945 issue, Chicago, Ill.

A. Singer, The Principles of Spray Rolling of Metals," Metals and h Iatcriuls, June 1970, pp. 246-250.

Primary I;'.\'umitlerC, W. Lanham Assistant E,\'untt'm'rD. C. Reiley, lll

Attorney, Agent, or FirmCooper, Duriham, Clark, Griffin 8-: Moran 57; ABSTRACT Aluminium alloy articles of high strength and high resistance to temperature softening are produced by spraying droplets of selected aluminium alloy en trained in a stream of gas onto a substrate under such Conditions that the metal droplets strike the substrate in a highly undercoolcd (supercooled) condition. On striking the substrate the undercooled liquid droplets flatten and are very rapidly chilled so that the alloying constituent is either maintained in supersaturated solid solution or is precipitated as a very fine precipitate. The deposit is consolidated by warm working. The selected aluminium alloy contains up to 25 percent of alloying constituent, which is in excess of the equilibr um solid solubility and has a low diffusion rate in aluminium. The preferred alloying constituents are one or more ofTi, V, Cr, Mn, Fe, Co, Ni, Zr, Nb and Mo. Si may be added but is unsuitable by itself. Particularly satisfactory results are obtained with eutectic ternary alloys containing Ni or Si.

8 Claims, 3 Drawing Figures PATENTED 3,899,820

sum 2 [1F 3 UL T/MA r5 7'NS/L STRESS MAI/m 5'0 Ibo 150 260 250 .360 350 460 HEAT TREA TMENT TEMPE/M TUR "c METHOD OF PRODUCING A DlSPERSlON-STRENGTHENED ALUMINUM ALLOY ARTICLE The present invention relates to the production of high hot strength aluminium alloy articles. In the process of the present invention these articles are produced by employing a spray casting process which is operated to produce a deposit of aluminium alloy droplets in which the alloying constituent. which may be one or more metals, is maintained either in supersatu rated solid solution or in the form of a precipitate of exceptionally fine particles. The mass of solidified droplets produced by the spray casting operation is then compacted to produce an article which may have high mechanical strength and resistance to temperature softening.

The spray-casting operation entails the atomisation and subsequent cooling of a stream of the molten alloy by high velocity jets of nitrogen or other suitable gas. In accordance with the present invention the atomised metal droplets are carried to a moving substrate where, upon impact. they become flattened and solidify almost instantaneously as a result of the initial gas cooling and secondary cooling from the substrate. The conditions for obtaining a supersaturated deposit or one containing a dispersion of fine lam) particles are:

If these conditions are satisfied then each droplet solidifies individually on deposition and the alloying additions. to be described. are largely retained in supersaturated solid solution or are dispersed as very fine lam) particles. The deposit that results from this operation is porous and of low strength and requires consolidation by hot working.

The object of the hot working (which may take the form of hot rolling. hot extrusion, hot pressing, hot forging or explosive forming) is as follows:

a. to heat porosity b. to break down the original boundaries of the flattened droplets so as to weld them together c. to cause some precipitation of very fine particles of the appropriate intcrmetallic phase (including metastable phases). the nature of which is dependent upon the alloy composition.

The effect of both (a) and (b) is to improve homogeneity and hence to improve ductility and strength. The effect of (c) is to produce a structure containing a very fine dispersion of very small intcrmetallic compound particles and it is this structure that determines the exceptional combination of properties. viz. high mechanical strength. high resistance to temperature softening and high elastic modulus.

The choice of alloying elements is determined by the need to achieve (a) maximum properties as previously defined. and (b) ease of spray-casting and consolidation.

According to the present invention a method of producing an aluminium alloy article having a high hot strength comprising establishing a substantially homogeneous body of molten metal comprising aluminium and 0.05 to 25 percent of alloying constitutent. the amount of said alloying constituent being in excess of the equilibrium solid solubility, the maximum value of the equilibrium solid solubility being 2 percent. said alloying constituent having a low diffusion rate in aluminium, establishing a stream of droplets of said molten metal in a stream of gas, preferably nitrogen or argon, or in some cases air, said droplets having an average diameter in the range of 50 am to l mm. projecting said droplets against a substrate, undercooling said droplets of molten metal by at least 50C during flight so that on striking the substrate they are very rapidly solidified to maintain said alloying constituent in supersaturated solid solution or in the form of particles of a size not greater than 1 am in said aluminium. and compacting the mass of droplets by hot working at a temperature in the range of 200 to 500C. Preferably the projection of the droplets is performed under such conditions that the droplets are undercooled by about 200C. Under these conditions it is estimated that the individual droplets are chilled at a rate of at least 10 C/sec. and indeed up to or even beyond 10 C/sec. in their transition from the liquid state to the solid state on striking the substrate.

Preferably the droplets are projected against a substrate. which presents a surface moving relatively to the source of said droplets so as to form a strip of cohered solidified droplets on the substrate. The strip is then preferably separated from the substrate for compaction.

The alloying constituent is preferably constituted by one or more of the following elements Ti. V, Cr. Mn. Fe, Co. Ni. Zr. Nb and Mo. It is preferred that at least one element should be present in approximately the amount required to form a eutectic with aluminium.

The alloying additions are chosen so as to have l very little solid solubility in aluminium and (2) very slow rates of diffusion in aluminium. up to at least 400C. These requirements ensure that the precipitates. when formed, are stable and resistant to change at elevated temperatures. The alloying elements of interest are principally the transition metals Ti. V. Cr. Mn. Fe. Co. Ni. Zr. Nb and M0. The total addition of one or more of these metals is preferably 3-15 percent. The liquidus temperature of the resulting alloy should not exceed approximately l.500C; otherwise the amount of heat required to be extracted by the gas be comes excessive to ensure rapid solidification of the droplets on impact on the substrate.

In addition to the elements listed above. the presence of other alloying elements. especially silicon at about l2 wt when added with one or more of the other elements provides for a strong alloy by making available increased numbers of siliconbearing intcrmetallic compound particles. even though silicon by itself is quite unsuitable because of its high rate of diffusion. For example, the addition of 127! Si to an Al-4 7( Fe alloy in creases the strength by increasing the number of intermetallic particles.

Aluminium alloys of near eutectic composition, such as alloys containing 1271 Si or 6% Ni are especially favourable alloys for spray-casting. because the liquidus temperature is relatively low. An alloy containing 5-7% Ni. preferably 5.5 6.5% Ni. with additions of one or more of the previously mentioned transition elements is particularly favourable. 1n consequence, the amount of heat needed to be extracted from the droplets by the gas to achieve undercooling is kept to a minimum. ln addition. the contraction stresses during solidification appear to be low. since deposits of these a1- loys have less tendency to curl-up from the substrate during spray casting.

lt has been observed that these desirable features (low liquidus temperature and flat deposit) are largely retained in ternary or higher order alloys that contain either 12% Si or 6% Ni even though the other elements, when present in binary alloys, cause difficulties by producing pronounced curling of the deposit.

It has already been proposed to produce aluminium alloy products of high strength by splat-casting, in which procedure metal droplets are subjected to rapid cooling by being projected against a chilled substrate. The metal particles are not subjected to any substantial degree of undercooling during flight. with the result 7 that nearly the whole of the initial heat content of the droplets is absorbed by the substrate. 1n consequence, it is only possible to build up a thin deposit of droplets having a desirable fine precipitate of insoluble alloying constituents. Any attempt to build up a thicker layer of metal by splat-casting results in a relatively coarse precipitate. As a result splat-casting may only be employed to form a layer of particles which are continuously re moved from the substrate and which are subsequently formed into products by powder metallurgy.

1n contradistinction the method of the present invention allows the formation of a relatively thick deposit. for example up to 2 cms thickness. of droplets having a high content of alloying constituents in supersatu- However this can only be achieved by correct adjustment of the relation between the gas supply and the supply of molten metal to the spray jet so as to ensure that the metal droplets impinge on the substrate in a suitable undercooled (but still liquid) condition. If the metal droplet temperature is too high or striking the substrate. the droplets will not instantly solidify on impact and solidification will be delayed. This will be evidenced by a relatively coarse structure in the deposit.

On the other hand. if the droplets are subjected to excessive cooling by the gas a large number of generally spherical solidified droplets will be present in the deposit, indicating solidification before impact with the substrate. Such droplets exhibit a coarse structure as they have not undergone very rapid chilling at the transition from the liquid to the solid state. By contrast, droplets deposited under correct conditions appear flattened as a rsult of their impact with the substrate and are relatively free from coarse second phase particles and indeed may exhibit a virtually featureless structure.

Thus a visual inspection of the deposit under a micro scope of suitable power will permit a determination of the correctness of the spraying conditions.

1n a series of experiments designed to spray various molten aluminium alloys under the conditions already designated above, a series of binary and ternary alloys were sprayed onto a substrate, which was either aircooled (A.C.) or water-cooled (W.C.). The loosely compacted strip formed by the spray casting process was then consolidated by warm-rolling at a temperature in the range of 250-450C. The consolidated strip was then further reduced by cold-rolling. The mechanical physical properties of the strip in the consolidated condition and in the eo1drolled condition were then measured. These. together with the temperature of the melt before spray casting and the warm-consolidation and cold-rolling conditions. are recorded in the followrated solution or in the form of a very fine precipitate. ing Table 1.

TABLE I Spray- Alloy casting Fabrication Roll-consolidation Cold-rolling WC. Melt Strip Ori Final Total No. Final Total C omposor temp. temp. ginal thickreducof thick reducition A.(. thick ness tion passes ness tion "C "C ness mm mm 7r mm /r 250 H125 1.73 33 7 0.47 73 W.C 750 300 9.9 10b 79 6 0.5) 72 350 10.5 1.13 80 0.55 74 (1.0%? Ni 400 10.5 2.12 80 5 0.55 74 300 1%.(1 1.68 79 (L47 72 AC. 800 350 7.6 1.67 78 (1 (1.44 74 400 1l.).l [.84 82 0.44 76 11.0% Ni A.( 920- 400 7.85 1.75 78 b (1.45 74 1000 450 7.30 1.77 76 ().45 54)? Co A.( 1000 400 8.00 1.82 77 8 0.46 75 450 8.25 1.81 78 (1.46 74 5.2% Mn WL J40 400 7.07 1.55 7H 7 0.40 74 450 7.35 1.30 Kl 03b 73 5.371 Mn A.(. 975 400 7.4 1.66 78 7 0.45 73 450 7.4 um 78 (1.44 73 (1.0)? Ni WIT. 920- 350 12.2 10h X3 9 0.58 72 3.592 Fe J80 400 ll.9(l ll l 82 (i (1.56 73 6.8% Ni A.(. v 350 13.25 3.72 7) H 1.()l 63 7 General observations relating to the effect of spraycasting and fabrication conditions on tensile properties can be made from Table l:

a. Within the temperature range for adequate roll consolidation, strength increased with drop in rollconsolidation temperature. This was probably due to a combination of work-hardening and a minimum of coarsening of the precipitate.

b. Elongation tended to increase with rise in rollconsolidation temperature.

c. Cold-rolling the roll-consolidated material tended to increase its strength and decrease the elongation, the amount varying from alloy to alloy. There was in the case of 5.47: Co a slight apparent softening of the material on cold-rolling.

Although ternary alloys have been shown in the main to be preferable to binary alloys, it may be that small additions of many elements would be preferable to larger additions of a few, as in this way a larger total fraction of alloying elements can be added before the liquidus temperature becomes too high for successful spraycasting.

An important point concerning spray-cast alloys of the present invention is that they are suitable for being based on relatively impure aluminium metal. Any socalled impurities will be taken into solid solution during spray-casting and will add to the general strength of the fabricated material. This is demonstrated by the good properties of the spray cast alloys containing iron and silicon, which are the two most common elements to be found in low purity aluminium.

in a further series of tests the effect of temperature on the hardness of spray-cast and consolidated alloy articles of the present invention was measured and compared with the effect of temperature on Dl9S alloy (corresponding to BS. [472: HFlo). having the following composition: 2.5/( Cu, l.55"/( Mg. 1.1% Fe, L2 2 Ni and 0.0671 Ti. the remainder commercial purity aluminium. The DI 9S alloy has the best resistance to temperature-softening of any normally employed, wrought commercial aluminium alloy.

In the accompanying FIGS. 1 and 2, the hardness and ultimate tensile stress of test pieces at room tempera ture. after being held at the indicated temperature for one week. is recorded. The test pieces were made by the spray-casting procedure as detailed above and are contrasted with the DlJS alloy in the fully aged (T6) condition. The are also contrasted with a spray-cast. roll-consolidated Al-l 2' Si alloy (which does not fall within the scope of the in\cntion I. It is to be seen that the binary and ternary spray-cast. roll-consolidated alloys. based on the transition elements. the substantiall} more resistant to softening b exposure to temperatures in the Zllll --lll(l( range for one week than is the knoun DI JS alloy The Properties shown in Table l and in l-KiQ. l and 2 do not necessarily represent the ultimate that might be achieved even for alloys with the same composition.

in the following Table 2 is set out the \alues for clastic modulus and density after lOll cUllSfllldtllltill obtained for certain of the alloys shown in FIGS. and 2 in contrast to Dl9S alloy (ill 8 TABLE 2 Alloy Elastic Modulus Density Composition GN/m Mg/m 6 Ni 79 2.84 5 Co 78 2.86 5 Mn 87 2.80 4 Fe 79 278 I2 Ni 87 3.02 6 Ni 4 Fe 87 2.92 4 Fe 12 Si 79 2.78 Conventionally prepared 79 2.78 Dl9S alloy One form of apparatus for carrying out the process of the invention is illustrated in FIG. 3 of the accompanying drawings.

The molten alloy, which is to be spray-deposited, is fed continuously to a crucible I, having a delivery tube 2. The delivery tube 2 has a bore of 3 mms and is surrounded by an array of twelve gas nozzles 3, each 2 mms in diameter. Nitrogen is delivered to the nozzles 3 from a supply pipe 4 at a pressure of about 80 p.s.i. (about 5.6 atmospheres).

The substrate in the form of a steel strip was positioned at a distance of about 14 ins. (350 mms) from the delivery tube 2 and advanced at such a rate as to build up a deposit about 20 mms thickness. Under these conditions the droplet size of the metal (considered as a spherical droplet in flight) was about l8() microns and it is estimated to have been cooled from an initial temperature of 850C by between 300 and 360C before striking the substrate so as to result in undercooling by l l()l7()C. The chilling of the molten droplets by the nitrogen gas is mostly effected in the immediate vicinity of the delivery tube nozzle 2, where the gas is coolest and the difference in velocity between the gas and the molten metal is highest. With a given apparatus the particle size of the metal droplets and the extent of the undercooling can be varied by increase or decrease of the gas delivery pressure. As already stated, the correctness of the undercooling of the droplets can be judged by visual inspection of the deposit and appropriate corrections may be made as necessary to im prove the deposit.

In order to demonstrate the importance of establishing the desired degree of undercooling during flight the alloy Al-Ni-(r'-FeAJ was sprayed under two different conditions 1 a I under the same conditions as in the foregoing examples. employing it cu.ft. of nitrogen per lb. of metal sprtncd l.l m /kg] and (h) -lll cuft. of nitrogcn per lb. of metal sprayed (0.5 Hi ni /Kg). In the latter case it was estimated from examination of the structure of the deposit that the droplets had been subiected to little (if an I undercooling before striking the substrate because the deposit did not e\hihit the ap pearance of a lay er of solidified flattened droplets and. as a consequence of this lack of undercooling. the pre cipitatc was coarse in character and the strength ofthe deposit after hot compaction and cold rolling was relatiyely poor as indicated in the folloning Table 3.

TABLE 3 Tensile Approx. properties of rolled sheet gas/metal 0.2% Proof U.T.S. Elong. Structure ratio Strtm eufijlbwt. MN/m MN/m Good 18 550 583 5 Bad 8-10 4| 5 2 We claim:

1. A method of producing an aluminium alloy article having a high hot strength comprising establishing a substantially homogeneous body of molten metal comprising aluminium and 0.05 to 25 percent of alloying constituent, the amount of said alloying constituent being in excess of the equilibrium solid solubility, the maximum value of the equilibrium solid solubility being 2 percent, said alloying constituent having a low diffusion rate in aluminium, establishing a stream of droplets of said molten metal in a stream of unheated gas, said droplets having an average diameter in the range of pm to 1 mm, projecting said droplets against a substrate, undercooling said droplets of molten metal by at least 50C during flight so that on striking the substrate they are very rapidly solidified to maintain said alloying constituent in supersaturated solid solution or in the form of particles of a size not greater than 1 pm in said aluminium, and compacting the mass of droplets by hot working at a temperature in the range of 200 to 500C.

2. A method according to claim 1 in which the droplcts are undercooled by l10-l70C.

3. A method according to claim 1 in which the alloy contains a total of 3-15% of transition metals Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb and Mo.

4. A method according to claim 3 in which the alloy contains up to 12% Si.

5. A method according to claim 1 in which the alloy is near eutectic composition for ease of spray casting.

6. A method according to claim 5 in which the alloy contains about 6% Ni or 12% Si.

7. A method according to claim 3 in which the alloy is a ternary alloy.

8. A method according to claim 7 in which the ternary alloy contains 5-77: Ni. 

1. A METHOD OF PRODUCING AN ALUMINIUM ALLOY ARTICLE HAVING A HIGH HOT STRENGTH COMPRISING ESTABLISING A SUBSTANTIALLY HOMOGENOUS BODY OF MOLTEN METAL COMPRISING ALUMINIUM AND 0.0 TO 25 PERCENT OF ALLOYING CONSTITUENT, THE AMOUNT OF SAID ALLOYING CONSTITUENT BEING IN EXCESS OF THE EQUILIBRIUM SOLID SOLUBILITY, THE MAXIMUM VALUE OF THE EQUILIBRIUM SOLID SOLUBILITY BEING 2 PERCENT, SAID ALLOYING CONSTITUENT HAVING A LOW DIFFUSION RATE IN ALUMINUM, ESTABLISHING A STREAM OF DROPLETS OF SAID MOLTEN METAL IN A STREAM OF UNHEATED GAS, SAID DROPLETS HAVING AN AVERAGE DIAMETER IN THE RANGE OF 50 UM TO 1 MM, PROJECTING SAID DROPLETS AGAINST A SUBSTRATE, UNDERCOOLING SAID DROPLETS OF MOLTEN METAL BY AT LEAST 50*C DURING FLIGHT SO THAT ON STRIKING THE SUBSTRATE THEY ARE VERY RAPIDLY SOLIDIFIED TO MAINTAIN SAID ALOYING CONSTITUENT IN SUPERSATURATED SOLID SOLUTION OR IN THE FORM OF PARTICLES OF A SIZE NOT GREATER THAN 1 UM IN SAID ALUMINIUM, AND COMPACTING THE MASS OF DROPLETS BY HOT WORKING AT A TEMPERATURE IN THE RANGE OF 200* TO 500*C.
 2. A method according to claim 1 in which the droplets are undercooled by 110*-170*C.
 3. A method according to claim 1 in which the alloy contains a total of 3-15% of transition metals Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb and Mo.
 4. A method according to claim 3 in which the alloy contains up to 12% Si.
 5. A method according to claim 1 in which the alloy is near eutectic composition for ease of spray casting.
 6. A method according to claim 5 in which the alloy contains about 6% Ni or 12% Si.
 7. A method according to claim 3 in which the alloy is a ternary alloy.
 8. A method according to claim 7 in which the ternary alloy contains 5-7% Ni. 